For a number of years the coating industry has been engaged in substantial developmental programs in the quest for procedures which would allow the coating of substrates at high production rates with the coating being cured to a tack-free condition at a speed commensurate with the contiguous processing steps. The industry has desired to eliminate the volatile solvents required in many of the well-known coating processes because of potential hazards or because of the cost of equipment to handle the evolved solvent vapors. In addition, the industry has been seeking coating formulations which would produce coatings which were durable and which would permit substantial additional processing of the workpiece, such as metal forming operations where the substrate is metal strip or container bodies, blanks and closures, or where plastic strip and paperboard are coated and formed.
Epoxy coating formulations have long been recognized as affording desirable properties in the finished coating, especially the toughness to withstand further processing. However, the problem has remained to develop a low cost epoxy coating formulation which would combine the desired rheological properties for the coating application with both reasonable pot life and rapid curing in the production line.
In Schlesinger U.S. Letters Pat. No. 3,708,296 granted Jan. 2, 1973, there are disclosed photopolymerizable epoxide formulations containing diazonium salts as photoinitiators which polymerize rapidly upon exposure to electromagnetic radiation to provide durable coatings.
In Watt U.S. Letters Pat. No. 3,794,576 granted Feb. 26, 1974, there are described desirable epoxy formulations which combine the desired rheological properties with suitable pot life and rapid curing at ambient temperatures, by incorporation of a photoiniator and at least about 15 percent by weight of an epoxidic ester having two epoxycycloalkyl groups. However, such esters do materially increase the cost of the formulation as compared with the more conventional epoxide prepolymer materials.
Since the disclosures of Schlesinger and Watt, a number of patents and publications have appeared proposing various photoinitiators for the epoxy formulations which could replace the diazonium catalysts specifically described in the Schlesinger and Watt Patents. Among these are the sulfonium salt catalysts disclosed in Smith U.S. Pat. No. 4,069,054 granted Jan. 17, 1978; Barton U.S. Letters Pat. No. 4,090,936 granted May 23, 1978; and Crivello U.S. Letters Pat. Nos. 4,069,055 granted Jan. 17, 1978 and No. 4,058,401 granted Nov. 15, 1977.
Generally, these sulfonium compounds have the structural formula shown and described in Crivello U.S. Letters Pat. No. 4,058,401 at column 2, lines 1-20: EQU [(R).sub.a (R.sup.1).sub.b (R.sup.2).sub.c S].sub.d.sup.+ [MQ.sub.e ].sup.-(e-f)
where R is a monovalent aromatic organic radical; R.sup.1 is a monovalent organic aliphatic radical selected from alkyl, cycloalkyl and substituted alkyl; R.sup.2 is a polyvalent organic radical forming a heterocyclic or fused ring structure selected from aliphatic radicals and aromatic radicals; M is a metal or metalloid; Q is a halogen radical; a is a whole number equal to 0 to 3 inclusive; b is a whole number equal to 0 to 2 inclusive; c is a whole number equal to 0 or 1; and where the sum of a+b+c is a value equal to 3 or the valence of S; d=e-f; f=valence of M and is an integer equal to from 2 to 7 inclusive; e is greater than f and is an integer having a value up to 8. Both General Electric Company and Minnesota Mining and Manufacturing Company have made efforts to commecialize catalyst systems using what appear to be mixtures of triarylsulfonium hexafluorophosphate compounds, but which are generally identified as triphenylsulfonium hexafluorophosphate.
Although this class of compounds is an effective photoinitiator, the range of ultraviolet radiation to which the triphenyl sulfonium hexafluorophosphate compounds are sensitive is somewhat limited. More recently, it has been found that bis-[4-(diphenylsulfonio)phenyl]sulfide bis-hexafluorophosphate compounds are effective photoinitiators with a broader range of spectral sensitivity in the ultraviolet range. The method of making such photoinitiators and the use thereof in the cationic polymerization of epoxy compounds are disclosed and claimed in Watt U.S. patent application Ser. No. 20,240 filed Mar. 14, 1979, and Chang U.S. patent application Ser. No. 20,514 filed Mar. 14, 1979, now U.S. Pat. No. 4,197,174, granted Apr. 8, 1980.
However, both of the above classes of sulfonium salt photoinitiators suffer from a common problem, namely, the evolution of malodorous organic sulfur compounds when the polymerizable compositions are subjected to electromagnetic radiation to effect decomposition thereof. These compounds are believed to be organic sulfides or mercaptans, and the term "organic sulfur compound" is used herein to generically encompass all such malodorous organic sulfur compounds resulting from the photodecomposition of sulfonium salt photoinitiators. This malodorous condition may have been sufficient to discourage more widespread adoption of such photoinitiators in commercial processes.
Accordingly, it is an object of the present invention to provide a novel photoinitiable cationically polymerizable composition utilizing sulfonium salt photoinitiators, which composition evidences clearly reduced or eliminated malodorous properties.
It is also an object to provide such a composition which is relatively stable during extended periods of storage and which will nevertheless polymerize rapidly to produce coatings with desirable mechanical properties.
Another object is to provide a novel polymerization process using such a composition and which is adaptable to a wide variety of high speed coating lines and which does not require extensive or expensive equipment to effect polymerization of the applied coating.